1. Field of the Invention
This invention relates to a rotary pulse motor most suitable for FA (Factory Automation) machine such as an industrial robot for which a relatively large torque is required.
2. Description of the Prior Art
FIG. 1 shows a first prior art rotary pulse motor which this Applicant previously developed. A cylindrical rotor 1 is made of magnetic material. Slit-like grooves 2b, 2b--are formed at an angular regular pitch in the peripheral portion of the rotor 1.
Thus, teeth 2a, 2a--are formed between the slit-like grooves 2b, 2b--, respectively. Permanent magnets 3, 3--are inserted into the slit-like grooves 2b, 2b--. A shaft 4 is fixed to a central hole of the rotor 1, and it is rotatably supported.
Pairs of magnetic poles (6U, 6U), (6V', 6V'), (6W, 6W),(6U', 6U'),(6V, 6V) and (6W', 6W') are formed in symmetry with respect to the axis or shaft 4, in the inner peripheral portion of a cylindrical stator 5 made of magnetic material. Not-shown coils are wound on the magnetic poles 6U to 6W'.
Pulse currents are, in turn, supplied to the coils. The rotor 1 is rotated stepwisely. The principle of the operation of the rotary pulse motor is described in detail, in the Japanese Patent Application No. 260923/1989.
Generally, such a rotary pulse motor is controlled in an open-loop manner, and so can position apparatus with high accuracy. Accordingly, it is used for driving a carriage in a printer which is one example of the FA machine. However, the torque is not so large as satisfactory. It is not suitable for the FA machine such as industrial robot for which a relatively large torque is required. The reason is as follows The teeth 2a, 2a extend in radial directions. Accordingly, areas of magnetic paths through which magnetic fluxes H flow, are narrower towards the center of the rotor 1 or the shaft 4, as shown in FIG. 2. The portions 7 of the teeth 2a, 2a, 2a--facing to the inner ends of the permanent magnets 3 are narrowest. The amount of the magnetic flux is limited. It cannot be larger than the saturable flux amount. Thus, a torque cannot be larger than the torque corresponding to the saturated flux amount.
FIG. 3 shows a second prior art rotary pulse motor which was developed by this Applicant and of a three-phase type and an inner-rotor type.
A cylindrical rotor 11 made of magnetic material is fixed to a shaft 16 at its central hole. As the first prior art rotary pulse motor of FIG. 1, teeth 12a, 12a--and slit-like grooves 12b, 12b--are alternately formed at an angular regular pitch in the peripheral portion of the rotor 11. Permanent magnets 13, 13--are inserted into the slit-like grooves 12b, 12b--.
Six magnetic poles 14A, 14B, 14C, 14A', 14B', 14C' are formed at an angular regular pitch in the inner peripheral portion of a cylindrical stator 14 made of magnetic material. The magnetic poles 14A', 14B' and 14C' are excited in opposite polarity to the magnetic poles 14A, 14B and 14C, respectively. Coils 15A, 15B, 15C, 15A', 15B' and 15C' are wound on the magnetic poles 14A, 14B,--14C', respectively. Magnetic teeth 14Aa, 14Ba, 14Ca, 14A'a, 14B'a and 14C'a are formed in the top end portions of the magnetic poles 14A, 14B, 14C, 14A', 14B' and 14C', respectively. Pulse currents in opposite polarities are supplied to the coils 15A, 15A' for A-phase and A'-phase, coils 15B, 15B' for B-phase and B'-phase and coils 15C and 15C' for C-phase and C'-phase.
However, there is the same problem in this rotary pulse motor, as in the first prior art rotary pulse motor of FIG. 1.
As shown in FIG. 4, paths for magnetic fluxes H are narrower towards the axis or shaft 16 of the rotor 11. Thus, an obtained torque not so large as satisfactory.
FIG. 5 shows a third prior art rotary pulse motor which was developed by this Applicant and of three-phase type and outer-rotor type. Magnetic teeth 22a, 22a--are formed at an angular regular pitch in an inner surface of a cylindrical rotor 21 made of magnetic material. Magnetic poles 25A for A-phase, 25B for B-phase, 25C for C-phase, 25A' for A'-phase, 25B' for B'-phase and 25C' for C'-phase are formed at an angular regular pitch in a peripheral portion of a cylindrical stator 24 made of magnetic material. Coils 26A, 26B, 25C, 26A', 26B' and 26C' are wound on the magnetic poles 25A, 25B, 25C, 25A', 25B' and 25C', respectively. The magnetic poles 25A', 25B' and 25C' are energized in opposite polarity to the magnetic poles 25A, 25B and 25C.
Teeth 28, 28--and slit-like grooves 29, 29 are formed alternately at an angular regular pitch in top end surfaces of the magnetic poles 25A, 25B, 25C, 25A', 25B' and 25C', respectively (FIG. 6). Permanent magnets 23 are inserted into the slit-like grooves 29 so that the teeth 28, 28 are polarized alternately in opposite polarity. A shaft 27 is fixed to the stator 24 at the central hole.
Pulse currents are, in turn, supplied to the coils 26A, 26B, 26C, 26A', 26B' and 26C'. The secondary rotor 21 is rotated stepwisely by the well-known principle which is described in detail, in the Japanese Patent Application No. 301965/1988.
This prior art rotary pulse motor has the same disadvantage as the above-described rotary pulse motors of FIG. 1 and FIG. 3.
As clearly shown in FIG. 6, the teeth 28, 28--and the permanent magnets 23, 23 --extend in the radial directions. Accordingly, the areas of the magnetic paths are narrowest at the portions 28' of the teeth 28, 28 --corresponding to the inner ends of the permanent magnets 23, 23--. The amounts of the magnetic fluxes are limited there. When the amount of the magnetic flux is larger there than a predetermined value, the magnetic fluxes are saturated. A larger torque cannot be obtained.
Further, since the six coils are required in accordance with the number of the magnetic poles, the weight of the rotary pulse motor is large, and its cost is high.
It is troublesome to mount the coils arround the magnetic poles, and it requires much labor and time.
Since the space factor cannot be large, the winding operation of the coils requires much time and labor.